1. Technical Field
The present invention relates to methods and apparatus for welding sheet metal in general, and in particular to automated methods and apparatus for welding discrete sheet metal bodies.
2. Background Information
It is known to weld sheet overlaps with welding rollers, especially with the aid of intermediate wire electrodes, such as in the welding of can bodies for example. A problem that arises especially when articles are welded individually, in particular can bodies where each body impinges individually on welding rollers in the horizontal position, as may be the case on manual welding machines or automatically fed welding machines, is that of welding the initial weld point at the start of the can body or other article to be welded. Because the welding energy or heat introduced at that point can only diffuse in one direction along the line of the seam, application of the full rated welding current for welding the overlapping seam at the initial point results in burning, or a defective welded joint. It is therefore known in the state of the art to reduce the welding current initially, as indicated in the schematic time/current diagram of FIG. 1. Accordingly, on the welding machine, the welding AC voltage or current and the drive to the welding rollers and/or wires are activated at a time Pt. 1 with the welding current source impressing a reduced welding current with a current value A which is lower than the rated current value B for the overlapping seam. The welding current 1 may for example be a 50 Hz alternating current.
At a time Pt. 2, the travel carriage of the welding machine, which carries the article to be welded e.g. a can body 2, reaches a predetermined position in front of the welding rollers, whereupon a time interval t1 commences at the end of which the rated current value B is impressed at the electrode wires by the welding current source. The article, in this case the body 2, reaches the electrode wires at a random point in time with respect to the welding current 1, e.g. as shown, when the current half-wave is at its peak, and welding commences at the reduced current A and changes over to the rated current B after the time interval t1. It is also known to lower the current towards the back end of the article, e.g. to the value C, as the back end of the article or can body has already been heated by a forward diffusion of heat through the article or body. In FIG. 1 this is indicated by time Pt. 3, the start of a time interval t2 governed by the position of the carriage and effecting a reduction of the current to the value C at the back end of the welded article. After the welded article has run through, the movement of the rollers and/or wires is stopped, and the welding current switched off, at time Pt. 4. FIG. 2 shows this known process for the case where coated, e.g. zinc galvanized, leaded, or aluminized, sheets are to be welded. These require higher welding currents than uncoated sheets, because of the presence of the coating. The higher welding current if applied as an uninterrupted alternating current can cause burning, and for this reason it is known to activate the welding current in bursts and to provide pauses between bursts (the so-called “stitch weld” process). FIG. 2 shows such a process with a current/pause ratio of 2:1 as an example. In other respects the conditions are the same as in FIG. 1: Pt 1 is the starting-point for the movement of the wires and the impression of the reduced current A, Pt. 2 defines the starting-point for the time interval t1, and is triggered by the position of the carriage, and at the end of t1 the rated current value B is impressed. Time Pt. 3, again governed by the position of the carriage, initiates the time interval t2, which triggers the reduction of the current to the final value C, and at time Pt. 4 the machine is stopped. Usually when the reduction is made to the current value C there is a changeover to uninterrupted alternating current, as shown in FIG. 2. Again in the case of the stitch weld process, the point on the current curve at which the leading edge of the article to be welded impinges on the welding rollers and/or electrode wire is undefined. In FIG. 2 this is shown as happening by chance at the point where the welding current passes through zero, but it could happen anywhere within a welding current burst or a welding current pause. Thus, although it is true that in the state of the art the initial weld point is welded at reduced current A, the weld quality at the initial point may vary. When the leading edge of the article to be welded impinges on an intermediate wire electrode that is already live, and therefore hot, a wire break may occur, especially if at that moment the intermediate wire electrode is being subjected to a peak current level (FIG. 1). The electrode is particularly susceptible to such damage when thick sheets, with a thickness of e.g. 0.5 to 1.25 mm, are being welded, where with a single overlap the material to be welded has twice the thickness, and with a folded or doubled overlap, four times the thickness.